Aircraft electrical systems are normally protected from high current and electrical shorts by circuit breakers. These circuit breakers are designed to remove power from a protected electrical circuit if an electrical current above a preset value is passed through the device. These high currents may occur for a number of reasons, such as a failure occurring in a piece of electrical equipment or damage occurring in a section of wiring insulation allowing the conductor to come into electrical contact with the structure of the aircraft, which is normally at ground potential.
Due to the nature of the circuit breaker mechanism, the tripping (opening) of the circuit breaker is not “instantaneous”. Some types of circuit breakers are allowed to continue supplying current for up to 40 seconds at twice their specified trip currents. An overload of five times the rated current may be allowed to flow for up to three seconds before a trip must occur. The trip delays are allowed because these devices rely on the overcurrent to heat up a bimetallic strip that functions as the detection element within the circuit breaker.
Circuit breakers conforming to these requirements have been used in aircraft for many years. Under normal operating conditions and under normal fault conditions, they operate satisfactorily. However, there are some fault conditions where the tripping delay greatly affects the ability of the circuit breaker to protect life and property. For example, certain types of wiring failures allow for a fault to ground which is not a “dead short”, meaning a direct, virtually zero-resistance electrical connection to ground. Some types of wiring insulations arc track when electrically faulted, which locally turns the material from an insulator to a conductor. An arc-tracked wire can be shorted to ground through a resistance which serves as a current limiter, which in turn allows the current to flow through the wire to the fault for some time until the circuit breaker is tripped. Until that occurs, the high current flow can damage and arc track other wires, adding their electrical supplies into the fault. This fault may initiate a cascading chain reaction which quickly compromises the safety of the aircraft.
These types of events occur sufficiently often in aircraft wiring systems that there is a need for a device which can detect arcing faults and remove electrical power from the protected electrical circuit more quickly than can a standard circuit breaker, while at the same time not tripping if an arc occurs in normal operation such as the arc that may occur when a switch is closed. This need is particularly acute for protecting DC electrical circuits, as well as low-frequency AC and AC electrical circuits generally. In addition, the device must meet other requirements, such as space limitations, low cost, and “invisibility” to normal operating conditions of aircraft electrical systems. The present invention fulfills this need, and further provides related advantages.